1. Field of the Invention
This invention pertains to control rod drive mechanisms for a nuclear reactor and more particularly to a drive mechanism which moves the control rod in a step-by-step manner by magnetic coils and which is especially adapted for rapid refueling of a nuclear reactor.
2. Description of the Prior Art
In a commercial nuclear reactor, heat, from which electricity is generated, is produced by fissioning of a fissile material such as enriched uranium. This fissile material, or nuclear fuel, is typically contained within a core made up of a plurality of fuel elements, coextensively arranged in a spaced parallel array. Movable control rods are dispersed throughout the core to control the fission process. The control rods generally comprise a plurality of elongated rods containing neutron absorbing materials, for example silver, indium and cadmium, which fit in openings among the fuel elements so as to be guided thereby during movement into and out of the core. Inserting a control rod into the core adds more absorber material and hence, decreases the nuclear reaction; conversely, withdrawing a control rod removes absorber material and hence, increases the nuclear reaction and thereby, the power output. The nuclear core and the control rods are positioned within and supported by a reactor vessel through which a reactor coolant flows.
Movement of the control rod into or out from the nuclear core is accomplished by control rod drive mechanisms which are mounted onto the top cover of the reactor vessel. Typically, a reactor pressure vessel is pressurized to a relatively high internal pressure. Therefore, to avoid large forces associated with high pressure drops and to avoid the use of high pressure seals, the control rod drive mechanisms operate at the pressure which exists within the reactor pressure vessel. Hence, the control rod drive mechanisms are housed within pressure bearing housings which for simplicity may be viewed as tubular extensions from the reactor pressure vessel. With this type of arrangement, it is common practice to add a missile shield above the control rod drive mechanisms for safety reasons. In the unlikely event of a major break of the pressure containing portion of the mechanisms, the missile shield would intercept parts of the drive mechanism that would be projected from the pressure vessel in missile like fashion.
In todays large commerical nuclear power plants, one of the more commonly used types of control rod drive mechanisms is referred to as the "magnetic jack" type of mechanism. With this type of mechanism, the control rods are jacked into or out from the nuclear core in a series of motions each involving moving the control rod a discrete incremental distance. The jacking movement is accomplished by two sets of axially spaced magnet coils in conjunction with magnet plungers having gripper arms attached thereto by alternately and sequentially gripping, moving and releasing the control rod. One example of this type of control rod drive mechanism is shown and described in U.S. Pat. No. 3,158,766, entitled "Gripper Type Linear Motion Device," filed Apr. 30, 1962, by Erling Frisch and assigned to the present assignee.
In this art, two magnet coils provide when energized, engagement of the control rod drive shaft by two sets of axially spaced grippers. A third magnet coil achieves lifting of the control rod. A load transfer function whereby the load is transferred from one gripper to the other before disengagement of the gripper initially supporting the drive shaft and a control rod pull down function are achived in this art by resilient means such as springs. The resilient means serve to bias the grippers a predetermined direction to accomplish the load transfer and pull down functions which had been accomplished in earlier art by two additional magnet coils.
In the type of nuclear reactor referred to herein, the nuclear fuel within the core must be replaced on the order of once every year. Core refueling involves removal of spent fuel assemblies and replacing these with new fuel assemblies. During refueling, the reactor is of course shut down and the pressure vessel is open so as to expose the core. Recent art has disclosed methods for rapidly refueling the core so as to minimize the down time of the nuclear reactor.
During rapid refueling all of the control rods are retracted or withdrawn from the nuclear core and stored within hardware which is attached to the closure head of the reactor pressure vessel and is commonly referred to, in the nuclear field, as the upper core support internals. At this time, the control rod drive shafts are fully housed within the pressure containing housing of the control rod drive mechanisms. Thus, with this arrangement, by simply removing the closure head of the reactor pressure vessel the core upper internals as well as the control rods are removed. Details of such an arrangement are disclosed in patent application Ser. No. 406,454 filed Oct. 12, 1973, entitled "Nuclear Reactor Internals Arrangement" by Erling Frishch et al.
Development of rapid refueling techniques and apparatus has revealed the need for utter reliability in the devices which hold the control rods in a retracted position within the upper internals during the refueling operations. Failure of the control rod holding devices at this time would result in the release of a control rod and cause a substantial increase in the refueling time. Such a failure would obviously negate the benefit sought to be achieved by the rapid refueling apparatus. Hence, means have been developed and disclosed whereby control rod drive mechanisms of the magnetic jack type are provided with safety latches which provide the control rods with an independent or redundant support. One such safety latch means is disclosed in U.S. Pat. No. 3,480,807, filed Apr. 11, 1967, entitled "Linear Motion Device," by R. E. Downs, et al. Downs discloses a safety latch which is pivotally mounted inside the housing of the mechanism. The latch is held in a non-latching position by a solenoid plunger to permit free movement of the element during normal reactor operation. The latch is spring biased to a latched position upon dc energization of the solenoid plunger so as to prevent the rod from moving in one of its two possible directions of movement.
While the Downs invention is not directed towards rapid refueling of a nuclear reactor, it is readily adaptable by one skilled in the art to such refueling methods. An arrangement is readily envisioned whereby the saftey latch is only activated when the control rods are fully contained within the upper internals. Should the main rod holding means fail at this time a projection on the safety latch which is in engagement with teeth on the control rod drive shaft would jam into position and effectively prevent the control rod from falling. However, the jamming aspects of such a safety means is not totally satisfactory. It is possible that the forces involved would cause physical deformation of either the teeth on the control rod drive shaft or of the saftey latch itself. Such physical deformation could severely hamper and even prevent future motion of the control rod. That is, when the reactor is reassembled upon the completion of refueling and placed back in power operation, control rod motion may be prevented resulting in loss of reactor control.
Another means for preventing release of the control rod during reactor refueling is disclosed in U.S. Pat. No. 3,766,006 filed July 8, 1970 entitled "Rapidly Refuelable Nuclear Reactor," by Erling Frisch. In this art, the missile shield previously referred to is moved to an upper position during refueling. This movement of the missile shield causes the holding electromagnetic means of the control rod drive mechanism to be replaced by permanent magnet means. After the refueling is completed, the missile shield is displaced downwardly to its normal position, replacing the holding permanent magnet means by the electromagnetic holding means which can then be readily deenergized. Hence, an electrical failure during reactor refueling would not result in release of a control rod. This arrangement is not totally satisfactory either. Major modifications are required of each control rod drive mechanism. Such modifications which include additional hardware, require space which may not be available in most reactor installations. Also, the modifications are quite expensive and complicated.
Therefore, there exists a continuing need to provide improved means for holding control rods of a nuclear reactor in a fully withdrawn position during reactor refueling.